The gunman, a man in his 70s whose name was not immediately released, was arrested without a struggle. An employee who works in the building said the gunman had a scheduled appointment with the slain doctor.

Police did not identify the person who was shot, but witnesses told NBC4 that the man was possibly a urologist.

Because the building is so new, witnesses initially thought the shooting was the sound of construction.

"I thought it was construction, like a nail gun," said Kristin Crotty, an employee at the medical facility.

The doctor has a wife and two children. His family declined to comment late Monday.